The present invention relates generally to the construction of audio power amplifiers, and more particularly to audio power amplifiers utilizing a power factor correction circuit.
Several United States Patents cover various aspects of power supplies, amplifiers, and power factor correction circuits. These include: U.S. Pat. No. 4,652,769 issued to Smith, et al. on Mar. 24, 1987; U.S. Pat. No. 4,677,366 issued to Wilkinson, et al. on Jun. 30, 1987; U.S. Pat. No. 4,808,946 issued to Carver, et al. on Feb. 28, 1989; U.S. Pat. No. 4,855,890 issued to Kammiller on Aug. 8, 1989; U.S. Pat. No. 4,961,044 issued to Kravitz on Oct. 2, 1990; U.S. Pat. No. 5,396,194 issued to Williamson, et al. on Mar. 7, 1995; U.S. Pat. No. 5,416,687 issued to Beasley on May 16, 1995; U.S. Pat. No. 5,510,753 issued to French on Apr. 23, 1996; U.S. Pat. No. 5,532,917 issued to Hung on Jul. 2, 1996; U.S. Pat. No. 5,543,753 issued to Williamson on Aug. 6, 1996; U.S. Pat. No. 5,606,289 issued to Williamson on Feb. 25, 1997; U.S. Pat. No. 5,767,744 issued to Irwin, et al. on Jun. 16, 1998; and U.S. Pat. No. 6,023,153 issued to Fink on Feb. 8, 2000. Each of these patents is hereby incorporated by reference.
U.S. Pat. No. 4,855,890, issued to Kammiller on Aug. 8, 1989 discloses a power factor correction circuit. In describing FIG. 4, the patent discloses that the capacitors for the power factor correction circuit may be switched into the circuit as the load increases. In describing FIG. 7B, this patent discloses a inductance connected in parallel with a triac such that the inductor of the power factor correction circuit may be removed from circuit as the load current increases leaving only the power factor correction capacitors. The switching of the capacitors is used to provide and change the amount of an initial half cycle charge and the switching of the inductors is used to slow and change the rate of current change to the vary overall the characteristics of the power factor correction over the load range.
U.S. Pat. No. 5,563,781, issued to Clauter et al. on Oct. 8, 1996 discloses a xe2x80x9cDual-Mode Power Converterxe2x80x9d for single-phase AC to DC power sources. This patent describes the use of a conduction mode and a switchmode power converter. As noted in Column 2, Lines 61-65, the switchmode power converter handles a fraction of the full output power and the peak power is conducted directly though the conduction-mode current path. In col. 3, lines 9-20, the patent describes how the control method is used to actively shut down the switchmode power converter when the power demand is above a specified level.
U.S. Pat. No. 6,023,153 issued to Fink on Feb. 8, 2000 discloses an xe2x80x9cAudio Amplifier Having Power Factor Correction.xe2x80x9d This patent teaches an audio amplifier including a very limited type of power factor correction using a power factor correction circuit for receiving the sinusoidal line voltage from the AC source and controlling the shape of the input current drawn from the AC source to correspond to the shape of the input voltage in order to minimize harmonic generation, the power factor correction circuit producing an intermediate source of voltage. As noted by the teachings of this patent, the power factor correction circuit must receive a sinusoidal line voltage and create a substantially higher intermediate voltage to feed the switching power supply.
Also of interest is Dierber, Kenneth W., Application Note, APT9502, Low Cost 1000 Watt, 300 Volt RF Power Amplifier for 13.56 Mhz, (RF Expo east 1995). This application note discloses that the DC to DC converter and the power supply operating from the AC mains is usually a significant cost of the overall system. Additionally, this application note discloses that power factor correction has been known as considered as being required due to the implementation of standards for equipment sold in Europe and expected to be implemented in the rest of the world.
Of additional note is Fairchild Semiconductor, Application Note 42030, Thoery and Application of the ML4821 Average Current Mode PFC Controller (Oct. 25, 2000). This application note discloses the use of fault control loops, with an example of one being used for overvoltage protection. On pages 9 and 10, a surge bypass diode is discussed to help to bypass surges at the input line during start-up to prevent the saturation of inductor L1.
These teachings fail to provide the advantages of the present invention. What is needed, then, is an Amplifier Having A Variable Power Factor.
The present invention is directed to an audio amplifier connected to the AC mains that is configured to efficiently use power factor correction by providing at least two levels of power factor control. One example of the multiple control levels is limiting the use of a power factor correction circuit to typical power draw conditions. The amplifier is designed to have a high power factor under typical power draw conditions by utilizing a power factor correction circuit. However, the amplifier limits the size and expense of the power factor correction circuit by allowing for a low power factor under full power conditions.
Since normal operation of the amplifier with audio program material will result in an output power of typically xe2x85x9 to ⅓ of the maximum output power, a first circuit can be used to achieve power factor correction and sized to provide high power factor only up to the typical power draw, and a low power factor high current rectifier is used when the amplifier is driven above the typical power level, as may occur during testing.
In one disclosed embodiment, the power factor correction circuit utilizes a constant sinusoidal current.
In another disclosed embodiment of the invention, the power factor correction circuit utilizes an intermediate cusp current.
In a still further one disclosed embodiment, the power factor correction circuit utilizes an intermediate sinusoidal current. In this embodiment of the present invention, an improved and consolidated design is provided by combining the cusp control method and sine wave modes of operation. In the first two described embodiments, the sine circuit low current power factor corrector operates with a sine output under all conditions and the cusp version operates with cusp current output that is shut down so that the output is supplemented by the bypass circuitry. The advanced version operates with the low current power factor corrector operating with a sine output at low currents and being overridden by the bypass circuitry to output a cusp output at high currents. The advanced circuit relies on maintaining a lower forward voltage drop across the bypass diode than across the series elements in the power factor corrector. This can be accomplished by using a diode with a higher inherent voltage drop in the power factor corrector, or by the use of a second rectifier in series with the power factor corrector.
Accordingly, it is an object of the present invention to provide improved amplifier designs utilizing a variable power factor.
Another object of the invention is the provision of an amplifier design which provides a high power factor under typical power draw conditions, and a low power factor under full power draw conditions.
Yet a further object of the present invention is the provision of an amplifier having a variable power factor utilizing a sinusoidal current circuit. Still another object of the invention is the provision of an amplifier having a variable power factor utilizing a cusp current circuit.
Other and further objects, features, and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the following disclosure when taken in conjunction with the accompanying drawings.